Boiler Short Cycling - Page 2

Xringer · 01-31-11, 07:14 PM

The way my Aquastat was wired, if there was an existing call for heat, within a certain boiler temperature range,
the burner would come on.

But, many times the T-Stat set-point would be reached a few seconds later and we get a short burn. About 2 to 20 seconds is typical.

Once the call for heat was gone, the Aquastat saw some level of heat and decided a burner run wasn't needed..
Sometimes, this would happen a few times a day.. It was random.

I've been using digital set-back T-stats for 20+ years now.
They aren't as important, after you retire.
Since, I might get up at 6AM or 10AM tomorrow..

wdrzal · 01-31-11, 09:58 PM

Quote:

Originally Posted by Xringer

The way my Aquastat was wired, if there was an existing call for heat, within a certain boiler temperature range,
the burner would come on.

But, many times the T-Stat set-point would be reached a few seconds later and we get a short burn. About 2 to 20 seconds is typical.

Once the call for heat was gone, the Aquastat saw some level of heat and decided a burner run wasn't needed..
Sometimes, this would happen a few times a day.. It was random.

I've been using digital set-back T-stats for 20+ years now.
They aren't as important, after you retire.
Since, I might get up at 6AM or 10AM tomorrow..

Xringer

I understand what your saying ,but for short cycling to occur a poor or not well thought out control strategy had to be used. In even 20 seconds there is no way to heat the exchanger(Fire box) walls up,the water(even a few degrees) and for the aquastat to sense the added heat from the burn that fast. something else is/was going on.

Does your duel fuel boiler you posted use a triple aquastat control box? What about multi-zones? I have seen some odd wiring control in that realm that allowed short cycling.

Odd most electronic T-stats lock out short cycling in the air conditioning mode but not in the heat mode.

Quote:

Xringer Once the call for heat was gone, the Aquastat saw some level of heat and decided a burner run wasn't needed..

I would bet this was the heat anticipator in the T-stat. Through electrical resistive heating ,heat is added directly to the t-stat.

Since I don't know the exact wiring the anticipator is the most likely cause. Aquastats usually have a minimium 10 degrees built in between Close/open. Aquastats usually see a delay due contact surface,using a bulb,capillary tube and diaphram design which is filled with some refrigerant vapor which = at least a few seconds delay.

Xringer you know your system better than I do. I'm just giving the usual scenario.

Also they make digital T-stats that are NOT programmable, except to move the temp up or down. I agree the programmabe T-stats only save if you use them.

I could understand if it was a forced air system ,but not hot water.Water gains temperature slowly even with a oil flame heating it.

Depending on boiler volume it take xx minutes to raise the water temp xx degrees. Numbers depend on volume,firing rate,HX design and effiency. Fossil fuel Boilers should have ~10 degree temp minimium between on & off

Also it take xx (time/minutes) for the pump to circulate & the radiators heat to move across the room and reach a T-stat to turn if off. then the T-stat needs to cool enough to turn back on for the short cycle to occur.

The Above reasons lend it self to a T-stat , unless you have a aquastat thats failed. Then they usually they revert to their wired position N/O or N/C. Random coincidences can occur. And it's this intermittent stuff that drive one crazy trying to explain.

Walt

Xringer · 01-31-11, 10:49 PM

Even with the downstairs zone turned off, there are times when the burner starts up,
I can go to the T-Stat and turn it down, which turns off the burner.

That means, there is a good chance, whenever the burner comes on, the T-stat stops calling
for heat and stops the burn.
The heat from an earlier burn finally makes it into the hall and warms up the T-stat.
Or, in the morning, when the sun warms up the front rooms..

I don't think there is any anticipater heaters in these new digital set-back units. I have seen them in the old round Honeywell T-stats (mercury in glass type).

They do have electronic hysteresis to keep the burner from being clicked off and on quickly.

I don't worry about it anymore, since the Manager insures long clean burns.

As for the intermittent stuff that drove me nuts, it was caused by the installer electrician,
when he over-tightened the switch mounting screws and cracked the burner cut-off switch.
The switch worked fine, about 99% of the time. But, once in a while it would fail to stay closed.
It looked fine and worked so well, I never took the time to open the box
and do a close inspection. That was the last thing on my list.
It was a good day when I found that one.. The screws that go through those mounting tabs..
So tight, they bent the switch and broke it's back.

That thing was intermittent for about 30 years.. Never got real bad, just irritating as heck!

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01-31-11, 07:14 PM	#11
Xringer Lex Parsimoniae Join Date: Feb 2009 Location: Woburn, MA Posts: 4,918 Thanks: 114 Thanked 250 Times in 230 Posts	The way my Aquastat was wired, if there was an existing call for heat, within a certain boiler temperature range, the burner would come on. But, many times the T-Stat set-point would be reached a few seconds later and we get a short burn. About 2 to 20 seconds is typical. Once the call for heat was gone, the Aquastat saw some level of heat and decided a burner run wasn't needed.. Sometimes, this would happen a few times a day.. It was random. I've been using digital set-back T-stats for 20+ years now. They aren't as important, after you retire. Since, I might get up at 6AM or 10AM tomorrow..

01-31-11, 10:49 PM	#13
Xringer Lex Parsimoniae Join Date: Feb 2009 Location: Woburn, MA Posts: 4,918 Thanks: 114 Thanked 250 Times in 230 Posts	Even with the downstairs zone turned off, there are times when the burner starts up, I can go to the T-Stat and turn it down, which turns off the burner. That means, there is a good chance, whenever the burner comes on, the T-stat stops calling for heat and stops the burn. The heat from an earlier burn finally makes it into the hall and warms up the T-stat. Or, in the morning, when the sun warms up the front rooms.. I don't think there is any anticipater heaters in these new digital set-back units. I have seen them in the old round Honeywell T-stats (mercury in glass type). They do have electronic hysteresis to keep the burner from being clicked off and on quickly. I don't worry about it anymore, since the Manager insures long clean burns. As for the intermittent stuff that drove me nuts, it was caused by the installer electrician, when he over-tightened the switch mounting screws and cracked the burner cut-off switch. The switch worked fine, about 99% of the time. But, once in a while it would fail to stay closed. It looked fine and worked so well, I never took the time to open the box and do a close inspection. That was the last thing on my list. It was a good day when I found that one.. The screws that go through those mounting tabs.. So tight, they bent the switch and broke it's back. That thing was intermittent for about 30 years.. Never got real bad, just irritating as heck!